Pandas Dataframe: set maximum value in column

I'm trying to set a maximum value of a pandas DataFrame column. For example:

my_dict = {'a':[10,12,15,17,19,20]}
df = pd.DataFrame(my_dict)

df['a'].set_max(15)

would yield:

a
0 10
1 12
2 15
3 15
4 15
5 15

But it doesn't.

There are a million solutions to find the maximum value, but nothing to set the maximum value... at least that I can find.

I could iterate through the list, but I suspect there is a faster way to do it with pandas. My lists will be significantly longer and thus I would expect iteration to take relatively longer amount of time. Also, I'd like whatever solution to be able to handle